子文档

¥Subdocuments

子文档是嵌入其他文档中的文档。在 Mongoose 中，这意味着你可以将结构嵌套在其他结构中。Mongoose 有两种不同的子文档概念：子文档数组 和单个嵌套子文档。

¥Subdocuments are documents embedded in other documents. In Mongoose, this means you can nest schemas in other schemas. Mongoose has two distinct notions of subdocuments: arrays of subdocuments and single nested subdocuments.

const childSchema = new Schema({ name: 'string' });

const parentSchema = new Schema({
  // Array of subdocuments
  children: [childSchema],
  // Single nested subdocuments
  child: childSchema
});

请注意，填充的文档不是 Mongoose 中的子文档。子文档数据嵌入在顶层文档中。引用的文档是单独的顶层文档。

¥Note that populated documents are not subdocuments in Mongoose. Subdocument data is embedded in the top-level document. Referenced documents are separate top-level documents.

const childSchema = new Schema({ name: 'string' });
const Child = mongoose.model('Child', childSchema);

const parentSchema = new Schema({
  child: {
    type: mongoose.ObjectId,
    ref: 'Child'
  }
});
const Parent = mongoose.model('Parent', parentSchema);

const doc = await Parent.findOne().populate('child');
// NOT a subdocument. `doc.child` is a separate top-level document.
doc.child;

• 什么是子文档？
• 子文档与嵌套路径
• 子文档默认值
• 查找子文档
• 将子文档添加到数组
• 删除子文档
• 子文档的父级
• 数组的替代声明语法

什么是子文档？

¥What is a Subdocument?

子文档与普通文档类似。嵌套结构可以具有 中间件、自定义验证逻辑、虚拟以及顶层结构可以使用的任何其他功能。主要区别在于子文档不是单独保存的，而是在保存其顶层父文档时保存。

¥Subdocuments are similar to normal documents. Nested schemas can have middleware, custom validation logic, virtuals, and any other feature top-level schemas can use. The major difference is that subdocuments are not saved individually, they are saved whenever their top-level parent document is saved.

const Parent = mongoose.model('Parent', parentSchema);
const parent = new Parent({ children: [{ name: 'Matt' }, { name: 'Sarah' }] });
parent.children[0].name = 'Matthew';

// `parent.children[0].save()` is a no-op, it triggers middleware but
// does **not** actually save the subdocument. You need to save the parent
// doc.
await parent.save();

子文档与顶层文档一样有 save 和 validate 中间件。在父文档上调用 save() 会触发其所有子文档的 save() 中间件，对于 validate() 中间件也是如此。

¥Subdocuments have save and validate middleware just like top-level documents. Calling save() on the parent document triggers the save() middleware for all its subdocuments, and the same for validate() middleware.

childSchema.pre('save', function(next) {
  if ('invalid' == this.name) {
    return next(new Error('#sadpanda'));
  }
  next();
});

const parent = new Parent({ children: [{ name: 'invalid' }] });
try {
  await parent.save();
} catch (err) {
  err.message; // '#sadpanda'
}

子文档的 pre('save') 和 pre('validate') 中间件在顶层文档的 pre('save') 之前执行，但在顶层文档的 pre('validate') 中间件之后执行。这是因为 save() 之前的验证实际上是一个内置的中间件。

¥Subdocuments' pre('save') and pre('validate') middleware execute before the top-level document's pre('save') but after the top-level document's pre('validate') middleware. This is because validating before save() is actually a piece of built-in middleware.

// Below code will print out 1-4 in order
const childSchema = new mongoose.Schema({ name: 'string' });

childSchema.pre('validate', function(next) {
  console.log('2');
  next();
});

childSchema.pre('save', function(next) {
  console.log('3');
  next();
});

const parentSchema = new mongoose.Schema({
  child: childSchema
});

parentSchema.pre('validate', function(next) {
  console.log('1');
  next();
});

parentSchema.pre('save', function(next) {
  console.log('4');
  next();
});

子文档与嵌套路径

¥Subdocuments versus Nested Paths

在 Mongoose 中，嵌套路径与子文档略有不同。例如，下面是两个结构：一种将 child 作为子文档，另一种将 child 作为嵌套路径。

¥In Mongoose, nested paths are subtly different from subdocuments. For example, below are two schemas: one with child as a subdocument, and one with child as a nested path.

// Subdocument
const subdocumentSchema = new mongoose.Schema({
  child: new mongoose.Schema({ name: String, age: Number })
});
const Subdoc = mongoose.model('Subdoc', subdocumentSchema);

// Nested path
const nestedSchema = new mongoose.Schema({
  child: { name: String, age: Number }
});
const Nested = mongoose.model('Nested', nestedSchema);

这两个结构看起来很相似，并且 MongoDB 中的文档将具有与这两个结构相同的结构。但存在一些 Mongoose 特有的差异：

¥These two schemas look similar, and the documents in MongoDB will have the same structure with both schemas. But there are a few Mongoose-specific differences:

首先，Nested 的实例永远不会有 child === undefined。即使不设置 child 属性，你始终可以设置 child 的子属性。但 Subdoc 的实例可以有 child === undefined。

¥First, instances of Nested never have child === undefined. You can always set subproperties of child, even if you don't set the child property. But instances of Subdoc can have child === undefined.

const doc1 = new Subdoc({});
doc1.child === undefined; // true
doc1.child.name = 'test'; // Throws TypeError: cannot read property...

const doc2 = new Nested({});
doc2.child === undefined; // false
console.log(doc2.child); // Prints 'MongooseDocument { undefined }'
doc2.child.name = 'test'; // Works

子文档默认值

¥Subdocument Defaults

默认情况下，子文档路径是未定义的，除非你将子文档路径设置为非空值，否则 Mongoose 不会应用子文档默认值。

¥Subdocument paths are undefined by default, and Mongoose does not apply subdocument defaults unless you set the subdocument path to a non-nullish value.

const subdocumentSchema = new mongoose.Schema({
  child: new mongoose.Schema({
    name: String,
    age: {
      type: Number,
      default: 0
    }
  })
});
const Subdoc = mongoose.model('Subdoc', subdocumentSchema);

// Note that the `age` default has no effect, because `child`
// is `undefined`.
const doc = new Subdoc();
doc.child; // undefined

但是，如果将 doc.child 设置为任何对象，Mongoose 将在必要时应用 age 默认值。

¥However, if you set doc.child to any object, Mongoose will apply the age default if necessary.

doc.child = {};
// Mongoose applies the `age` default:
doc.child.age; // 0

Mongoose 递归地应用默认值，这意味着如果你想确保 Mongoose 应用子文档默认值，有一个很好的解决方法：使子文档路径默认为空对象。

¥Mongoose applies defaults recursively, which means there's a nice workaround if you want to make sure Mongoose applies subdocument defaults: make the subdocument path default to an empty object.

const childSchema = new mongoose.Schema({
  name: String,
  age: {
    type: Number,
    default: 0
  }
});
const subdocumentSchema = new mongoose.Schema({
  child: {
    type: childSchema,
    default: () => ({})
  }
});
const Subdoc = mongoose.model('Subdoc', subdocumentSchema);

// Note that Mongoose sets `age` to its default value 0, because
// `child` defaults to an empty object and Mongoose applies
// defaults to that empty object.
const doc = new Subdoc();
doc.child; // { age: 0 }

查找子文档

¥Finding a Subdocument

每个子文档默认都有一个 _id。Mongoose 文档数组有一个特殊的 id 方法，用于搜索文档数组以查找具有给定 _id 的文档。

¥Each subdocument has an _id by default. Mongoose document arrays have a special id method for searching a document array to find a document with a given _id.

const doc = parent.children.id(_id);

将子文档添加到数组

¥Adding Subdocs to Arrays

MongooseArray 方法（例如 push、unshift、addToSet 等）将参数透明地转换为其正确类型：

¥MongooseArray methods such as push, unshift, addToSet, and others cast arguments to their proper types transparently:

const Parent = mongoose.model('Parent');
const parent = new Parent();

// create a comment
parent.children.push({ name: 'Liesl' });
const subdoc = parent.children[0];
console.log(subdoc); // { _id: '501d86090d371bab2c0341c5', name: 'Liesl' }
subdoc.isNew; // true

await parent.save();
console.log('Success!');

你还可以使用文档数组的 create() 方法 创建子文档而不将其添加到数组中。

¥You can also create a subdocument without adding it to an array by using the create() method of Document Arrays.

const newdoc = parent.children.create({ name: 'Aaron' });

删除子文档

¥Removing Subdocs

每个子文档都有自己的 deleteOne 方法。对于数组子文档，这相当于在子文档上调用 .pull()。对于单个嵌套子文档，deleteOne() 相当于将子文档设置为 null。

¥Each subdocument has its own deleteOne method. For an array subdocument, this is equivalent to calling .pull() on the subdocument. For a single nested subdocument, deleteOne() is equivalent to setting the subdocument to null.

// Equivalent to `parent.children.pull(_id)`
parent.children.id(_id).deleteOne();
// Equivalent to `parent.child = null`
parent.child.deleteOne();

await parent.save();
console.log('the subdocs were removed');

子文档的父级

¥Parents of Subdocs

有时，你需要获取子文档的父文档。你可以使用 parent() 函数访问父级。

¥Sometimes, you need to get the parent of a subdoc. You can access the parent using the parent() function.

const schema = new Schema({
  docArr: [{ name: String }],
  singleNested: new Schema({ name: String })
});
const Model = mongoose.model('Test', schema);

const doc = new Model({
  docArr: [{ name: 'foo' }],
  singleNested: { name: 'bar' }
});

doc.singleNested.parent() === doc; // true
doc.docArr[0].parent() === doc; // true

如果你有深度嵌套的子文档，则可以使用 ownerDocument() 函数访问顶层文档。

¥If you have a deeply nested subdoc, you can access the top-level document using the ownerDocument() function.

const schema = new Schema({
  level1: new Schema({
    level2: new Schema({
      test: String
    })
  })
});
const Model = mongoose.model('Test', schema);

const doc = new Model({ level1: { level2: 'test' } });

doc.level1.level2.parent() === doc; // false
doc.level1.level2.parent() === doc.level1; // true
doc.level1.level2.ownerDocument() === doc; // true

数组的替代声明语法

¥Alternate declaration syntax for arrays

如果你使用对象数组创建结构，Mongoose 会自动将对象转换为结构：

¥If you create a schema with an array of objects, Mongoose will automatically convert the object to a schema for you:

const parentSchema = new Schema({
  children: [{ name: 'string' }]
});
// Equivalent
const parentSchema = new Schema({
  children: [new Schema({ name: 'string' })]
});

下一步

¥Next Up

现在我们已经介绍了子文档，让我们看一下 查询。

¥Now that we've covered Subdocuments, let's take a look at querying.